Plastic materials used for automobile bumpers, molds and so on, generally, have bad wetting for coating compositions such as paint compositions and primer compositions and are inferior for coating. Especially, as polyolefin resins such as a polypropylene resin do not have polar groups in molecules and are chemically inactive, the plastic materials of polyolefin resin materials are much inferior for coating and adhesion to coating compositions.
Since the coating compositions in which the medium is an organic solvent have problems in flammability and since their solvents have high VOC ratios coating compositions made of an emulsion with an aqueous medium have been developed. However, this emulsion is low in coating and adhesion performance in comparison with the coating compositions in which the medium is an organic solvent.
To solve the problem of coating with the emulsion, JP-A-05-214188 discloses raising polarity by chlorinating polyolefin which is the main material of the coating compositions. However, as polyolefin chloride has low cohesion, its adhesion is insufficient. Then, JP-A-03-182534 discloses to improve adhesion by grafting polyolefin chloride with unsaturated polycarboxylic acid and/or acid anhydride. However, grafting with unsaturated polycarboxylic acid and/or acid anhydride which have high water-solubility, produces another problem of lowering the water-resistance of films.
Polyolefin chloride has high hydrophobicity and it is therefore difficult to disperse in water stably. JP-A-06-080738 discloses a technique to polymerize a reactive emulsifier made of acrylic resin based polymer, and when needed, other reactive monomers in a water medium under the existence of polyolefin and to improve water dispersion stability of polyolefin. This technique is to improve water dispersion stability of the emulsion by covering a polyolefin with an emulsifier by means of using the reactivity of an acrylic resin based polymer. However, an acrylic resin based polymer which has high hydrophilicity and polyolefin which has low polarity are incompatible. Therefore, an emulsion composition stabilized by an acrylic resin based polymer emulsifier has two problems. The first is stability with the passage of time (storage stability) of the water dispersion, and the second is that a coated film obtained from this is muddy. It is therefore required to improve the water dispersion of polyolefin chloride by another means.
As for polyolefin chloride, since the levels of coated film strength and weather resistance are low, there are cases where it is necessary to improve coated film strength and weather resistance by using polyolefin chloride together with an acrylic resin or other resin. In this case, it is difficult to use these together because polyolefin chloride and an acrylic resin or other resin are incompatible. Then, JP-A-05-209006 suggests choosing an acrylic resin which has comparatively good compatibility and grafting polyolefin chloride with this acrylic resin. This technique has the desired effects of using polyolefin chloride and acrylic resin together by grafting the acrylic resin and, further, maintains compatibility between polyolefin chloride and the acrylic resin or other resin using the compatibility improving effect of a grafted acrylic resin. However, in the grafting of an acrylic resin to polyolefin chloride, as there is a quantitative limit to compatibility, it becomes impossible to increase the amount of an acrylic resin and, further, there are not so many kinds of (meth) acryl based oligomers which have good compatibility with polyolefin chloride.